1. Field of the Invention
This invention relates to a laser printer, and more particularly, to such a printer which is adapted to record a color image on a photographic film.
2. Description of the Prior Art
Laser printers are used in photography and in the graphic arts for printing on a receiving medium such as film. When such printers are used for color imaging, they generally include a separate channel for each of the primary colors. In U.S. Pat. No. 4,728,965, for example, there is disclosed a laser printer which includes three optical channels, and each channel includes a gas laser which projects a beam of intense coherent light at a predetermined wavelength. The intensity of the light beam in each channel is modulated by an acoustooptic modulator in accordance with an electrical signal representing image information for one of the primary colors. The three beams of light are combined by a beam combiner, and the combined light beam is scanned onto a receiving medium by a rotating polygon.
In recent years, there have been attempts to use diode lasers instead of gas lasers in laser printers. The use of diode lasers reduces the cost of the printer and permits a drastic reduction in the size and complexity of the printer. Diode lasers can be modulated directly at frequencies as high as several hundred MHz by simply modulating the drive current, and thus, no external modulators are required. Further, the low intrinsic noise of diode lasers makes it possible to eliminate the high-bandwidth servo controls used for noise cancellation in gas laser printers. There are, however, problems which have to be solved in using diode lasers in printers for the graphics arts. One of the problems is that available gallium aluminum arsenide diode lasers emit in the infrared. As a result, a special recording material which is sensitive to the infrared must be used, and the diode lasers in a multilaser system must be carefully selected to obtain the widest possible spectral separation between the lasers.
There are also optical problems involved in combining the diode laser beams for color systems and and in controlling the combined beam to obtain high-quality, continuous tone images. One of the factors which makes the control of the beams more difficult is that the optics must be confined to a relatively small space in the more compact diode laser printers. The optics must be arranged to focus the beam on the polygon to control facet pyramid error, and the optics must also control the size of the beam such that aberrations induced by the optics are not prohibitively large.